Disc drives of the type in which the present invention is particularly useful are well known in the industry, and are frequently referred to as "Winchester" disc drives.
Disc rives of this type include a sealed housing in which the principal components are enclosed. These components include a spindle motor having a hub on which one or more rigid discs are mounted for rotation. This spindle motor is usually of the brushless DC type and spins the discs at 3600 RPM or a similar high speed. Each disc surface contains a large number of circular, concentric data tracks on which data are recorded and from which these data can later be retrieved. The processes of recording and retrieving data are referred to as "writing" and "reading" respectively.
There is typically a single read/write head associated with each surface of each disc, although several designs have included more. The read/write heads are under control of electronic circuitry, allowing access to all of the data tracks.
These read/write heads are usually one of two types:
1. an interrupted ferrite ring wound with wires for inducing flux changes in the disc media to write data, and sensing changes in flux to read data, or;
2. an analogous arrangement formed using the thin-film deposition technology of the semi-conductor industry.
Whichever type of read/write head is used, the heads themselves are incorporated in or mounted to a sled-like structure called a slider. The slider typically has two elongated, flat lower surfaces called air bearing surfaces (ABS), analogous to the runners on a sled. These ABS are designed and dimensioned to "fly" the head over the surface of the disc on a thin layer of air dragged along by the rapidly spinning disc. Disc drives of the current technology fly the heads at a distance from the heads of less than ten microinches (0.000010 inch).
Because of the size of the flying height, any contaminant within the disc drive could disrupt the proper relationship between the head and the disc, and potentially cause permanent damage to the head and/or disc. Therefore, the discs and heads, plus the mechanisms for moving them, are sealed in a "bubble" to maintain the necessary degree of cleanliness.
This bubble is commonly formed by a base casting--to which are mounted the spindle motor and discs and the actuator mechanism for moving the heads sealed to a top cover. There is usually some sort of gasket between the base and the top cover to provide the needed cleanliness seal.
The top cover is usually secured to the base with a multiplicity of screws. In prior art applications, extreme care must be exercised to squeeze the gasket to a great enough extent to seal the drive and yet not so tight as to distort the gasket, which could result in excess leakage and thus contamination.
Another drawback of previous designs is that, when the top cover is securely screwed to the base, excessive distortional stresses can be exerted on the base, resulting in unwanted changes in the geometric relationship between the various sub-assemblies mounted to the base casting. These stresses can be especially detrimental when thermally induced expansion or contraction of the components occurs during the use of the disc drive.